a. FIGS. 1-8
Conventional impact tools such as jack hammers and pile driver typically use a combination of hydraulic and pneumatic fluids to operate the device. In the more common designs, the hydraulic and pneumatic fluids act against opposite sides of a common piston with the hydraulic fluid being used to restore (i.e., move the piston to compress the pneumatic fluid or gas) whereupon the device is fired and the piston is accelerated under the force of the compressed gas.
In the most common and perhaps simplest prior art design, hydraulic fluid under pressure is used to restore the piston and compress the gas whereupon the hydraulic fluid is then vented to initiate the power stroke (i.e., the portion of the cycle in which the piston is accelerated under the force of the compressed gas). However, the inherent viscosity and mass of the hydraulic fluid makes it resist rapid flow and acceleration with the result that undesirable back pressure is generated on the piston. This back pressure resists and thus reduces the piston's acceleration under the force of the compressed gas. Further, if acceptable rapid flow is obtained, the accompanying result is often the entrapment of air in the hydraulic fluid which reduces its performance characteristics. Attempts to overcome these problems generally rely upon reducing the volume of hydraulic fluid involved per cycle by reducing the effective piston area on which it operates. However, this reduction results in the need to use higher hydraulic pressures to obtain a given operating pressure for the compressed gas in order to achieve a desired acceleration of the piston. Alternately, a lower gas pressure can be used to achieve the desired acceleration but the piston area against which the gas operates must then be enlarged. Both solutions create undesirable side effects as in the first instance, the strength of the tool must be increased to handle the higher hydraulic pressures and in the second instance, the size of the tool must be increased which increases its weight and may well make the tool unwieldy. Other attempts to overcome the venting and acceleration problems of the hydraulic fluid during the power stroke include increasing the size of the hydraulic outlet at the vent and using valves that reduce the viscous losses associated with the rapid discharge of hydraulic fluid from the system; but, such solutions are often impractical because of the added cost, complexity, and weight to the tool.
In other common designs of impact tools, the problems associated with venting and accelerating the hydraulic fluid during the power stroke are avoided by venting the hydraulic fluid prior to the initiation of the power stroke. However, such alternate approaches have numerous drawbacks. Chief among these is the need to retain the piston at its restored position before firing while the hydraulic fluid is removed. This is traditionally done mechanically or by isolating the piston from the compressed gas until the firing step is initiated. To accomplish this, secondary hydraulic systems or other arrangements must be included in the tool to drain the hydraulic fluid prior to firing. Such retaining and draining mechanisms add complexity and weight to the tool not to mention the additional time that must be added to the cycle to accommodate the separate venting stage after the piston has been restored. Further, in some designs that retain the piston by isolating it from the compressed gas rather than mechanically holding it, efficiency is often reduced because in isolating the piston, a small volume of compressed gas becomes trapped and must be vented. Such venting not only wastes the energy of the trapped gas but also requires that additional gas periodically be added to the system.
Examples of these and other prior art approaches include U.S. Pat. Nos. 4,181,183 to Okada, 3,878,019 to Reynolds, 3,872,934 to Terada, 3,866,690 to Lance, 3,735,823 to Terada, 3,267,677 to Bollar, and 3,205,790 to Bollar.
In a marked change from these prior art approaches, the present inventor earlier invented a design which avoided the hydraulic fluid problems outlined above. In his prior approach, the present inventor provided a chamber with first and second members sealingly mounted in opposing outlets of the chamber for movement toward and away from each other along a common axis. In operation, the chamber was pressurized with a compressed gas and the first member was forced into the chamber until it contacted and made a face seal with the second member. Once the faces of the two members were in contact, the motion was stopped and a valve was opened and left open to vent the gas trapped between the two faces of the two members to the atmosphere. After a delay to insure the venting had been accomplished, the first member was then retracted wherein the second member followed because the effective area on the second member exposed to the compressed gas within the chamber was such as to create a net force in the direction of movement of the first member. To fire the device, the venting valve of the first member was then closed and a triggering valve in the first member was opened to allow compressed gas from the chamber to enter the volume between the sealed faces of the first and second members. As the pressure in this volume increased, the forces on the second member overcame the forces tending to hold the second member against the first and the faces separated. Once the faces separated, the face seal between them was broken and the force on the air piston due to the compressed gas accelerated the second member away from the first along the common axis.
Although this prior approach of the present inventor avoided the problems associated with venting and accelerating the hydraulic fluid in the previous designs of others, it too had several drawbacks. Specifically, everytime the device was restored, a volume of compressed gas was lost through the venting step to the atmosphere thereby requiring a supply of compressed gas to replenish the lost volume. This supply made the device impractical for many applications because of the need for a compressor to continually restore the lost gas. The operation also required the venting valve to remain open to the atmosphere during the restoring of the second member since even minor leaks in the face seal between the members or in the triggering valve would result in a premature firing of the device. Further, any such leakage in the face seal or in the triggering valve resulted in additional loses of compressed gas and a need for a larger supply of compressed gas to replace losses. Another drawback was that the face seal and the valve seals were exposed to the difference between the maximum operating pressure (e.g., 600-1000 psi) and atmospheric pressure during each cycle. Consequently, this high differential pressure required close tolerances and expensive, special valving and often resulted in unreliable operation at high pressures. The inclusion of two valves in the first member also required space allowances for the valve housing which increased size, weight, and complexity. Connectors for actuating the valves added even more complexity and weight. Finally, the pause or delay at the end of the stroke when the faces of the two members were sealed to allow for the actuation of the venting valve and the venting step itself made the device impractical for operation with electric or hydraulic motors and for high speed applications.
It was with the deficiencies and drawbacks of these prior art approaches in mind that the present invention was developed. With the tool of the present invention, the need to vent compressed gas to the atmosphere or outside the working chamber has been avoided thus eliminating the need to continually recharge the working chamber to compensate for the discharged gas. Further, in the preferred embodiments of the invention, the critical seals between the two main moving parts of the tool need only to be exposed to a minimal differential pressure to operate the tool wherein higher overall pressures can be used for maximum acceleration of the hammer member. Additionally, the need for providing elaborate and bulky valves on one of the two, main working members as in the inventor's prior approach has been eliminated reducing the cost, complexity, and size requirements of the tool. Also, the need for the pause or delay to properly seal the two working members prior to the restoring stroke of the inventor's previous device has also been eliminated making the new tool adaptable for operation by electric or hydraulic motors and for high speed applications.